1. Field of the Invention
The invention relates to a spark plug structure in use for internal combustion engine, and particularly concerns to a spark plug improved in heat-resistance and fouling resistance.
2. Description of Prior Art
In a spark plug generally used for internal combustion engine, there are provided a metallic shell having a male thread at its outersurface and an insulator into which a center electrode is placed. The metallic shell is made of steel carbide, while the insulator has been mainly made of alumina porcelain. The physical properties of these materials such as thermal conductivity, have been playing important roles in determining thermal characteristics of a spark plug. The characteristics represents heat-resistance which indicates preignition resistance at high temperature atmosphere, and at the same time, representing fouling resistance which indicates carbon formation at low temperature atmosphere.
Therefore, it has been desired to provide a performance-enhanced spark plug which is capable of complying with versatile demands with high output of recent engine and low fuel consumption.
Therefore, it is an object of this invention to provide a spark plug structure which is capable of avoiding preignition, and imparting good thermal transfer from an insulator to a metallic shell with good heat-resistance.
It is another object of this invention to provide a spark plug structure which is capable of determining greater insulation path by lowering the temperature of an insulator with improved fouling resistance.
It is further object of this invention to provide a spark plug structure which is capable of maintaining high mechanical strength and air-tightness.
According to the present invention, there is provided a spark plug structure comprising; a cylindrical metallic shell; a tubular insulator having a center bore, and a center electrode placed into the center bore of the insulator to form a spark gap with a ground electrode depending from the metallic shell; the metallic shell being made of material having a tensile stress of more than 40 Kg/mm.sup.2, and having a thermal conductivity of more than 60 W/m.multidot.k.
Various other objects and advantages be obtained by the present invention will appear in the following description and in the accompanying drawings.
According further to the invention, there is provided a spark plug structure comprising; a cylindrical metallic shell having a ground electrode at its front end which has a thermal conductivity of more than 60 W/m.multidot.k; an tubular insulator having a center bore, and at least a front end of the insulator having a good thermal conductivity of more than 60 W/m.multidot.k and placed into the metallic shell; a center electrode placed into the center bore of the insulator with a front end somewhat extended from that of the insulator; a terminal inserted into the center bore of the insulator in alignment with the center electrode; an electrically conductive glass sealant provided at an annular space between the insulator and the terminal, and one between the insulator and the center electrode; the ground electrode being made of nickel or nickel alloy, the ground electrode being connected to the metallic shell through a metallic ring which is made of different metal from the metallic shell such as steel, stainless steel or nickel alloy.